Can an audio amplifier be used to transmit or receive radio signals?

[ofosot69]

Is it possible to make an audio amplifier transmit radio signals when connected to its output with a ferrite coil or any other loop antenna and make the signal receive with another audio amplifier?

I have this question in my mind because of these reasons:

1. An audio amplifier can respond to frequencies from 20 Hz to 20 kHz and when connected with a speaker, the coil in the speaker resonates as per the output frequency and if that can be connected to a loop antenna instead, it should radiate radio waves too.

2. While experimenting with a tape recorder many years ago, I encountered one strange thing. While I was adjusting the screw of the tape head, the screwdriver accidentally touched one of the head connectors that was connected with the audio amplifier. And I was able to hear an AM radio station. I guess the screwdriver was working like a whip antenna and the tape head was working like a loading coil.

So, based on these facts, I am assuming that a audio amplifier is able to transmit or receive radio frequencies without the need of any carrier wave. This would be done using direct induction just like how a matching transformer works between an audio amplifier and a speaker.

 

[Nigel Goodwin]

Basically - NO - your entire premise is incorrect.

What can be done, and commonly is, is a loop system for deaf people.

You have a number of turns of wire around a room, perhaps three - using three core mains wire - you connect the speaker output of an amplifier to it, and the hearing aids contain magnetic receiver, allowing the deaf people to hear the audio sent to the loop.

But it's magnetic, not radio. Essentially it's a room sized transformer.

 

[schmitt trigger]

Tape recorders have a multi-kilohertz bias oscillator. It is high enough that it could have heterodyned a radio signal and created a modulation product very close to the audio baseband.

 

[Nigel Goodwin]

Tape recorders have a multi-kilohertz bias oscillator. It is high enough that it could have heterodyned a radio signal and created a modulation product very close to the audio baseband.

The bias oscillator only works on record, it's simpler than that - he was injecting largish RF signals into a very high gain amplifier, over loading it, causing demodulation.

 

[schmitt trigger]

You are right, tape head preamps had tons of gain.

 

[danadak]

Google "vlf amatuer radio", down to 3 Khz, fair amount of work and experiments
over the years.

Internet Archive: Digital Library of Free & Borrowable Texts, Movies, Music & Wayback Machine

archive.org

Regards, Dana.

 

[Papabravo]

You also need an insanely large antenna to radiate efficiently at such a low frequency.

 

[danadak]

For reception small works as well - https://www.pa3hcm.nl/?p=1232

Same for transmission - https://www.cdarc.org.uk/downloads/presentations/VLF_Through_Ground.pdf


Of course as stated not as efficient as wavelength sized solutions.

 

[Nigel Goodwin]

For reception small works as well - https://www.pa3hcm.nl/?p=1232

Same for transmission - https://www.cdarc.org.uk/downloads/presentations/VLF_Through_Ground.pdf


Of course as stated not as efficient as wavelength sized solutions.

Thanks for the links, really interesting

 

[Tony Stewart]

The distortion of front end high gain preamp common mode level with DC bias from an audio short may have caused preamp to be nonlinear like a diode to rectify the AM carrier so that faint audio signals of the strongest channel may be heard. This is not a reciprocal process to enable transmitting.

 

[Tony Stewart]

For reception small works as well - https://www.pa3hcm.nl/?p=1232

Same for transmission - https://www.cdarc.org.uk/downloads/presentations/VLF_Through_Ground.pdf


Of course as stated not as efficient as wavelength sized solutions.

Memories come back of my 1st project after BsEE univ. Watts all this Stuff about VLF?

I had to design a 5 ch. micropower Doppler Rx sniffing VLF signals broadcast by US military @ 1MW scatter around the northern hemisphere for submarine communication and self-tracking. They are cesium time-sync'd VLF carriers with a common denominator of 100 Hz to extend the range of tracking distance in km/360 deg cycle. It had to survive -40'C in the Beaufort Sea and only consume 10 mW max (or was it 50 mW? ) using my JFET to CMOS PLL design. I received the signals and filter then with 5 deg X cut ceramic filters then down-converted each of the 5 VLF channels to 100 Hz. The result was to track the weather station on ice movement of 20 km /day by measuring the relative phase shift with time interval counters.

My colleague designed the 400 MHz Tx for the uplink to GOES #1. It was the world's 1st floating automated weather station and also in Canada's Arctic. The idea was to understand the motion of ice and replace the daily reconnaissance airplane looking tracking icebergs and ice flows that could shear oil wells or drills from moving ice. Circa 1976. I It , ended up in Siberia long after I upgraded my salary 25% after 4 yrs moving from Aerospace R&D to Telecom R&D.

I wanted to make an electronic fuse using a tiny light bulb which acts as a more constant current limit with its higher voltage PTC characteristics but ended-up using a better current limiter design that would not break open and could self-recover in the event of a glitch from lightning. The antenna was too small as it was sitting near the 20m vertical egg-beater (custom Darrius/Sevonious wind gen) My Egyptian colleague who had a PhD but didn't know transistors as well designed the 800 W generator with hockey puck diodes for the battery piles and quickly learned about the misgivings of the SOA to fuse power transistors.

At that time 60 Hz from Manitoba's Dorsey HVDC station was also cesium time sync'd but I also had a 10 MHz OCXO counter to measure phase shift and the atmosphere carrier the signals around the ionosphere in the globe except for the daily diurnal shift of sunrise and sunset of the atmospheric path.

It was a blast, but only my boss's boss went to Tuktiactuk to measure the <10 uV signal strength I had to deal with. The problem with Aerospace is every design was a one-of-a-kind world's first and everything that went up or out the door never came back. It wasn't until 30 yrs later I met my goal of designing something in high-volume production 10k/mo for Lucent at half their best effort in cost and 1/4 of the time to 1st prototype (4wks) and half their size IU vs 2U 19" rack. Those were the days that carried into nights and the next day without breaks like Elon sleeping in the Tesla factory except I felt like a 1 man show. Although I had a brilliant mentor, a gold medalist who taught me all the BJT design options and I had a library of mil-stds to learn about everything with discrete components and Xtals.

But it was my Dad who gave me the electronic inspiration when I was 12 and he made me a Crystal detector pickle jar AM radio just like your tape recorder but no batteries required with a crystal ear bud that had more bass tightly fit before going to sleep. Thankyou Dad.